A variety of geodetic methods and geodetic devices have been known since antiquity for surveying a target point. In this case, the distance and angle from a measuring device to the target point to be surveyed are recorded as spatial standard data, and in particular the location of the measuring device is captured, in addition to possibly provided reference points.
A theodolite, a tachymeter, or a total station, which is also referred to as an electronic tachymeter or computer tachymeter, represents a generally known example of such surveying devices or geodetic devices. Such a geodetic measuring device of the prior art is described, for example, in published application EP 1 686 350. Such devices have electro-sensory angle measuring functions and possibly distance measuring functions, which permit a determination of direction and distance to a selected target. The angle or distance variables are ascertained in the internal reference system of the device in this case and must possibly still be linked to an external reference system for absolute position determination.
Modern total stations have microprocessors for the digital processing and storage of captured measurement data. The devices generally have a compact and integrated construction, wherein usually coaxial distance measuring elements and also computer, control, and storage units are provided in one device. Depending on the expansion stage of the total station, means are integrated for motorization of the target optics, for reflector-free distance measurement, for automatic target search and tracking, and for remote control of the entire device. Total stations known from the prior art furthermore have a wireless data interface for establishing a wireless connection to external peripheral components, for example, to a data capture device, which can be designed in particular as a handheld data logger, field computer, notebook, small computer, or PDA. By means of the data interface, it is possible to output measurement data, which are captured and stored by the total station, for external further processing, to input externally captured measurement data into the total station for storage and/or further processing, to input or output remote control signals for remote control of the total station or a further external component, in particular in mobile field usage, and to transfer control software into the total station.
The measurement precision achievable during the surveying operation varies depending on the embodiment of the target point to be surveyed. If the target point is represented, for example, by a target reflector—such as a 360° prism—designed especially for surveying, substantially more precise measurement results can thus be achieved than in the case of a reflector free measurement, for example, to a point of a house wall to be surveyed. This is because, inter alia, the emitted optical measurement beam has a planar beam cross-section rather than a punctiform beam cross section and therefore not only measurement radiation scattered on the actual target point to be surveyed is received, but rather also measurement radiation scattered from points in the immediate field of vision vicinity of the target point, to which the measurement radiation is also applied. For example, the roughness of the surface of the point to be surveyed influences the precision of reflector-free measurements in a known manner.
For aiming at or targeting a target point to be surveyed, surveying devices of the type in question have a targeting unit (such as a telescope). In a simple embodiment variant, the targeting unit is designed, for example, as a telescopic sight. Modern devices can additionally have a camera, which is integrated into the telescopic sight, for capturing an image, wherein the captured image can be displayed in particular as a live image on a display screen of the total station and/or a display screen of the peripheral device—such as a data logger—used for the remote control. The optics of the targeting unit can comprise a manual focus in this case—for example, a set screw for changing the focal position of the optics—or can have an autofocus, wherein the focal position is changed, for example, by servomotors. Automatic focusing units for telescopic sights of geodetic devices are known, for example, from DE 197 107 22, DE 199 267 06, or DE 199 495 80.
The optical system or the optical viewing channel of the targeting unit contains in particular an objective lens group, a focusing lens group, and an ocular, which are arranged in this sequence from the object side. The position of the focusing lens group is set in dependence on the object distance so that a sharp object image results on an optical element, which is arranged in the focal plane, having targeting marking (in particular reticle or graticule, or plate having crosshair marking and hash markings). This optical element having the image created in this plane can be observed through the ocular.
The coaxial camera (for example, having CCD or CMOS surface sensor), which is provided in addition to the direct vision channel, can be arranged in a further image plane provided in the telescope optics, for which decoupling of a partial light beam via a beam splitter can be provided, so that an image (or a series of images or a video stream) can be recorded through the objective using the camera.
Furthermore, an additional separate transmitting and receiving channel branch can be provided for the coaxial electronic distance measurement. In addition, common surveying devices currently comprise an automatic target tracking function (ATR: “automatic target recognition”), for which a further separate ATR light source—for example, a multimode fiber output, which emits light having a further defined wavelength—and a special ATR camera sensor are additionally integrated in the telescope.
To prevent distortions, color faults, or vignetting—i.e., a brightness drop in the edge regions of an observable field of vision—enormously high demands are placed on the individual optical components. Accordingly, special and complexly coated optics are generally required for decoupling and coupling individual wavelengths, wherein in spite of the coating, the visual band is to enable display with the best possible color fidelity. In addition, the high complexity of the telescope requires a high level of expenditure for the required high-precision mounting and alignment of the optical components.
For example, the construction of telescopic sights of the type in question of geodetic devices is disclosed in published applications EP 1 081 459 or EP 1 662 278.
In the case of a typical one-man surveying task using target reflector, for example, a total station is set up in the terrain. The user moves a handheld surveying rod, which supports the target reflector, to a target point to be surveyed, wherein the position of the target reflector and therefore of the target point can subsequently be determined as follows. The control of the total station is performed in particular by remote control by the user carrying the surveying rod by means of a data logger, which has a wireless connection to the total station. The data logger can be attached in this case to the surveying rod equipped with the target reflector or can additionally be handheld by the user in addition to the surveying rod.
Aiming at a target can generally be performed in this case either using the physical crosshair provided in the telescope/telescopic sight or by means of a live image, which is displayed to the user in the display screen of the base station (or the data logger), and an electronic crosshair overlaid thereon, which is provided by the camera arranged coaxially in the telescopic sight as a targeting unit of the total station. Accordingly, the user can appropriately align the total station on the desired target, which is recognizable in the live image, on the basis of the live image, for which an artificial (i.e., electronic) crosshair can be displayed superimposed in the displayed live image of the coaxial camera. The image position at which the electronic crosshair is to be displayed is to be selected as much as possible in this case so that the spatial direction thus indicated corresponds as accurately as possible to the direction which is indicated by the physical optical element, which is integrated in the telescope, having targeting marking (i.e., the reticle, for example). This is independently of whether the direction indicated by the reticle (targeting direction) itself also has an error in relation to the actual measurement direction (i.e., the direction in which finally the measurement radiation is emitted and therefore represents the measurement direction). This direction error between targeting direction and measurement direction is handled in this case independently of the problem of the positioning of the electronic crosshair in the image and is to be considered separately.
Since a physical recalibration (realignment) of the targeting direction indicated by the physical optical element having targeting marking, so that it corresponds to the measurement direction, can be complex and no differences are to exist in the case of targeting via looking through the telescope in comparison to targeting via observation of the display image having the artificial crosshair, in an ideal video total station, the artificial crosshair is to indicate as exactly as possible the same (targeting) direction as the physical targeting marking. To display the artificial crosshair as faithfully as possible at a corresponding point in the display screen image, a calibration (with determination of corresponding calibration parameters) is carried out at the factory after assembly of the surveying device. Such a factory calibration, as is known per se to a person skilled in the art in the field of surveying device construction, establishes a relationship between the measurement coordinate system of the surveying device and the camera coordinate system in consideration of the present surveying device geometry. Examples of such known factory calibrations are described, for example, in patent literature publications U.S. Pat. No. 7,982,866, U.S. Pat. No. 7,623,224, and EP 1 695 030, wherein, however, a procedure based on camera image recordings of known target marks, which is complex with respect to the required environment and the measurement conditions, is required. In this context, calibration parameters with respect to standing axis and tilt axis errors (direction errors) or a displacement of camera component parts can also be ascertained using such factory calibrations in a known manner.
However, such errors do not remain stable in the course of time. Thus, for example, they are influenced by physical shocks (for example, during transport), by temperature influences, or by other material properties, which vary in the course of time.
It is true that a field calibration, which can be carried out by the surveyor himself in the field, with respect to standing axis errors and tilt axis errors is often made in a known manner before performing a surveying task (for example, known as a two-location measurement or changeover measurement, in which an identical target is targeted successively once in a first location (face I) and once in a second, changed location of the telescope (face II) via the physical reticle and the angles are measured in each case). However, in addition, during later use of the display screen for targeting a target (i.e., the electronic crosshair), a deviation can result in the target coordinate determination in relation to targeting on the basis of the physical reticle.